1. Field of the Invention
The present invention relates to a three-dimensional balun, and in particular to a multi-layer microwave circuit that is implemented in high density microwave integrated circuits for linking an unbalanced circuit to a balanced circuit or vice versa.
2. The Related Art
Conventional microwave circuits or balanced-unbalanced converters are often referred to as baluns, commonly used in mixers, push-pull amplifiers, and voltage-controlled oscillators, and phase shifters, and antennas. A typical balun converts balanced signals to unbalanced signals, or from unbalanced signals to balanced signals in microwave or milliwave transmissions.
For instance, when signals close to the center frequency of the operating balun are input from the unbalanced side, the signals are converted and then output through two outputs having the same amplitude and producing 180-degree phase shifts.
The baluns can be distinguished by types. A wire-wound transformer provides an excellent balun covering frequencies from low kHz to GHz. An active type balun that incorporates a pre-amplifier is able to provide broad bandwidth and high gain, but often accompanied by high spurious output level and extra power consumption. A lumped type balun, though saves on circuit space employing lumped inductors and capacitors, is limited in bandwidth and can only support below 10 GHz.
A Marchand type balun provides large bandwidth, good isolation, and low spurious output level. The Marchand type balun is more tolerant of low coupling ratio in the even mode than the coupled line balun, and has a wider bandwidth. A rat-race coupler is commonly used for microwave frequencies with bandwidths up to 10–20%. However, the Marchand and rat-race baluns have to use quarter-wavelength transmission lines, thus taking up more circuit space.
In these aspects, the three-dimensional balun according to the present invention substantially reduces or obviates the limitations and disadvantages of the prior art. This new approach is to build three-dimensional baluns by laminating multiple microwave circuit layers on top of each other. The actual signal transmission lines are embedded in the first layer, while the upper layers are in a transmission line configuration.